In a cement or concrete mixture, both the plastic (fresh) state and the hardened state thereof are defined by the type and degree of attraction between particles. The “bleeding” process defined as the appearance of water accumulated on the surface of the materials and heterogenous separation (segregation) due to density difference of the aqueous phase and the aggregate, is precisely largely conditioned by the type of attraction between particles. The main objective of a specific type of flocculating additives (or thickeners) is precisely to minimize or reduce this heterogenous phase separation by means of altering the attractive forces between the particles, which affects the colloidal state of the aqueous phase of the cement. It must be taken into account that “bleeding” affects the durability, the mechanical strengths and the possibilities of applying the material (Josserand et al. Cement and Concrete Research 2006; 36(9):1603-1608).
It is thought that material flocculation or thickening process occurs due to the interaction of highly charged groups in the molecular chain of the flocculating agent with cement particles. The chains are adsorbed on those particles, binding them and giving rise to an attraction between particles. The cohesion increases, slump flow in flow table test decreases and “bleeding” is reduced. The effects on mechanical strengths under compression closely depend on the dose of flocculant or thickener used (Ramachandran, Concrete Admixtures Handbook, “2nd Edition, Noyes Publications, New Jersey, 1995).
Particularly, in the context of rendering or spread mortars, it is necessary for the material to have a consistency such that it does not flow through the vertical face, and furthermore it usually requires a substantial shortening of setting times which forces the use of setting accelerating additives for certain applications as spread mortars for tunnels.
This second type of compounds reduce setting times and accelerate the development of early mechanical strengths, which entails advantages with respect to the times for executing the work, putting into service, reducing the risks due to frost, etc.
The joint solution for both requirements (thickening the fresh cement mixture and accelerating the setting times) is usually achieved by means of using two or more different additives. Some of the additives used especially as setting accelerants have problems associated with their use, for example, the corrosion of metal reinforcements incorporated to the cement system when calcium chloride is used as accelerant; or the high level of soluble salts incorporated to the mixture, given the generally high dosages necessary for some of these products (Justnes and Nygaard, Cement and Concrete Research 1995; 25(8):1766-1774; Aiad et al., Cement and Concrete Research 2003; 33:9-13; Aggoun et al. Construction and Building Materials 2008; 22:106-110).
In turn, polyelectrolytes which have been occasionally used as thickeners or flocculants either have environmental repercussions due to their origin and hazardous effect on the ecosystems (such as polyacrylamide or polyvinyl polymers, for example) or the origin thereof does not contribute to sustainable construction industry development (such as cellulose derivatives of plant origin and the inherent risks of deforestation).
Attempts have been made to find additives which overcome the preceding problems and which to the extent possible act as multipurpose additives, for example as an additive which is both a flocculant and a setting accelerator, in order to reduce the use of different additives in the preparation of cementitious compositions.
One of the additives which have been used is chitosan. Chitosan is the product from the N-deacetylation of a natural biopolymer, chitin. It consists of glucosamine and N-acetylglucosamine units bound by 1-4 glycosidic bonds. It is therefore a chitin derivative, a natural biopolymer, the second most abundant biopolymer after cellulose, found in the exoskeletons of invertebrates such as crustaceans and insects. Chitosan has good solubility in different acidic solutions and is insoluble in neutral and alkaline aqueous solutions. The term chitosan is used in a generic manner to designate any glucosamine and N-acetylglucosamine-based copolymer, regardless of its molecular weight (50 to 2000 kDa) and degree of deacetylation (40-98%), even though it has been confirmed that the degree of deacetylation and the molecular weight determine the properties of the polymer. Chitosan has a good resistance in alkaline media, is flexible and has high resistance to heating due to the intramolecular hydrogen bonds formed between the hydroxyl and amino groups (Muzzarelli, C., and Muzzarelli, R. A. A, Journal of Inorganic Biochemistry 2002; 92(2):89-94).
BRPI0600628, WO86/00291, JP52022026 and JP2001316162 are some of the documents describing the use of chitosan as an additive in binder compositions for construction. However, they face the problem of the extremely poor solubility of this compound especially at the strongly basic pH of such compositions.
Document BRPI0600628 describes a cementitious composition suitable for constructing oil wells comprising chitosan in a proportion of 1-5%. The addition of chitosan to the matrix of Portland cement confers it greater mechanical strength and reduces the probability of cracking under severe temperature conditions.
Application WO 86/00291 describes cementitious compositions comprising a flocculating agent selected from a list of polymers among which chitosan is included. Nevertheless, the preferred embodiment contemplates the use of carboxymethylcellulose.
Document JP52022026 describes a cementitious composition comprising Portland cement and chitosan. The product molded with the composition described in this document has a bending strength greater than that achieved for the product molded in the absence of chitosan.
Document JP2001316162 describes a method for increasing the strength of a calcium silicate board for walls and ceilings which comprises adding an acidic solution of chitin and chitosan to the base material when curing in an autoclave.
The use of chitosan as an additive in binder compositions for construction poses a problem: the insolubility of non-modified chitosan at strongly alkaline pH of the binder matrix with Portland cement base, limiting its action on the aqueous phase of the mixture and therefore its action on the rheology in the mixtures in fresh state.
There is also a need to develop new multipurpose additives which improve the properties of the already existing additives and which act on more than one of the main functions defined for other additives, for example, that the additives meet the double objective of functioning as flocculating agents and setting accelerators. Furthermore, it is desirable that said additives can be incorporated in the lowest doses possible in the matrix of the cementitious compositions. Additionally, it will also be highly desirable for them to be non-caustic, which contributes to occupational safety and health.
The present application is based on the inventors confirming that the incorporation of carboxymethylchitosan, a carboxymethyl-substituted derivative of chitosan, as an additive in binder compositions (such as for example cement mortars or the like) results in a double effect:                a clear thickening (flocculating) effect of the system in fresh (or plastic) state, a reduction in the slump flow value in the flow table (a test defined in the UNE-EN standards for evaluating the consistency of a mortar in fresh state), with reductions, and therefore thickenings of up to 50% depending on the dose, being observed.        a considerable setting acceleration in these mixtures, with already studied reductions in workability time of up to 98% (from 322 minutes, with a 0.5 water/cement ratio for an additive-free mortar, to 7 minutes of workability time for mortars with 0.5% additive by mass with respect to the cement).        
Furthermore, this double effect is clearly shown starting from very low doses of carboxymethylchitosan, especially if compared with the necessary doses of other additives to achieve similar effects (approximately 4-6% in the case of aluminates and 8-12% in the case of silicates, always with respect to the cement/binder weight. Rey A., Spread Concrete: Dosage, Manufacture and Implementation, 1st Conference on Spread Concrete (Hormigón Proyectado: Dosificación, Fabricación y Puesta en Obra, I Jornada sobre Hormigón Proyectado), on line, www.comerciallinas.com/PDF/Sikaproyectado.pdf) and in the absence of chlorides. Therefore, some of the advantages of the present invention are: reduction in raw material consumption, cost optimization and reduction in risks derived from using corrosive chemical products at work (transportation restrictions, storage condition limitations and the risk of accident due to exposure to chemical products through respiratory pathway and or skin are reduced).